1. Field of the Invention
The present invention applies to the field of determining timing for received signals in radio communications systems and, in particular, to determining timing using a training sequence having a periodic structure.
2. Description of the Prior Art
Mobile radio communications systems such as cellular voice and data radio systems typically have several base stations in different locations available for use by mobile or fixed user terminals, such as cellular telephones or wireless web devices. Each base station typically is assigned a set of frequencies or channels to use for communications with the user terminals. The channels are different from those of neighboring base stations in order to avoid interference between neighboring base stations. As a result, the user terminals can easily distinguish the transmissions received from one base station from the signals received from another. In addition, each base station can act independently in allocating and using the channel resources assigned to it.
Such radio communications systems typically include a broadcast channel (BCH). The BCH is broadcast to all user terminals whether they are registered on the network or not and informs the user terminals about the network. In order to access the network, a user terminal normally tunes to and listens to the BCH before accessing the network. It will then use the information in the BCH to request access to the network. Such a request typically results in an exchange of information about the network using separate control and access channels and ends in the user terminal receiving an assignment to a particular base station.
While frequency and timing offset can sometimes be determined by a user terminal based on the BCH, the initial request for access is typically transmitted by the user terminal without any knowledge of its distance to the base station, its timing delay or the direction to the base station. In a spatial diversity multiple access system, the base station can enhance the capacity of the system by determining the position and range to the user terminal as well as any other spatial parameters. The timing uncertainty of the arrival time of such request messages are proportional to the round trip delay encountered by messages traveling between the base station and the mobile terminal. For systems with a high coverage area per base station, this range and therefore the delay uncertainty may be very large.
In order to accurately resolve the access request and determine spatial parameters, the timing or delay, frequency offset and spatial signature of the user terminal""s message are all desired. Typically, a long training sequence must be accurately found and determined in an extended analysis window. A search of all possible starting or ending locations for the sequence over the entire delay spread can create great demands on the processing resources of a base station and increase the amount of time required to generate a reply to the access request.
A method and apparatus are provided that allows timing to be determined in two steps. According to one aspect of the present invention, the invention includes searching a received digital signal burst for a known repeating core sequence, the core sequence having a first symbol sequence, and, upon finding one of the core sequences, determining sequence timing of the found core sequence using the found core sequence. The invention further includes searching the received burst for a marker sequence following the repetition of the core sequence, the marker sequence having a second symbol sequence different from the first symbol sequence and upon finding the marker sequence, determining timing of the received digital signal burst using the found marker sequence.